Josephson Weak Links Research Articles

Fabrication and characterization of ramp edge-type junction and SQUIDs

We have investigated the properties of high-T/sub c/ ramp-edge geometry Josephson weak links using superconducting YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) electrodes and PrBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//(PBCO) normal-metal layers. Unambiguous evidence of Josephson behaviour has been observed. Current-voltage characteristics (CVC) were qualitatively consistent with the resistively-shunted-junction (RSJ) model. Well developed Shapiro steps for 8.9 GHz microwave irradiation have been observed. Furthermore we have fabricated dc-SQUIDs with PBCO barriers that modulate up to a temperature of 80.6 K. At 4.2 K we find a flux-voltage modulation depth of /spl ap/36 /spl mu/V. >

Recovery of superconductivity and recrystallization of ion-damaged YBa 2Cu 3O 7− x films after thermal annealing treatment

We have systematically studied the annealing effects of high-quality epitaxial YBa 2Cu 3O 7− x (YBCO) films on (100) SrTiO 3 substrates implanted with 100 keV Ar ions at different fluences. The mechanisms for T c degradation induced by ion implantation have been discussed. After annealing in oxygen atmosphere in a temperature range from 450 to 950°C, R( T) curve measurements show a partial or complete recovery of superconductivity. The recrystallization of ion-implanted YBCO films have been investigated using X-ray diffraction (XRD) and Rutherford backscattering spectrometry with channeling. It is found that the annealing at 450°C indeed results in the movement of oxygen defects and ordering of the oxygen sublattice in the sample implanted with low to moderate fluences, however, only a small partial recovery of superconductivity is observed. The results suggest that ion implantation induced T c degradation is mainly caused by disordering of the cation sublattice not the oxygen sublattice. After annealing at 850°C, the recovery of superconductivity and recrystallization of implanted YBCO films are related to the fluence. For YBCO films implanted with low to moderate fluences (1 × 10 11−1 × 10 14/cm 2), the superconductivity can be nearly completely recovered, and the pure c-axis orientation of the samples is observed in the XRD spectra. However, recrystallization of amorphous YBCO films implanted with high fluences (1 × 10 15−1 × 10 16/cm 2) is quite different. The orientation of the samples depends on the annealing temperature. With the increasing of the annealing temperature from 750 to 950°C, the regrowth orientation gradually changes from a- to c-axis orientation. After annealing at temperatures of 750 and 800°C, the samples have the nearly pure a-axis orientation. At 850°C, the sample has a mixed a- and c-axis oriented structure but the a-axis orientation is still dominant. Above 900°C the structure of the annealed sample has the pure c-axis orientation. In the case of amorphous YBCO films caused by high-fluence Ar ion implantation, the T c0 is lower than 77 K in the whole annealing temperature range due to the polycrystalline structure. The experimental results indicate that the amorphous YBCO films formed by ion implantation regrow homogeneously rather than epitaxially and thus have the polycrystalline structure. This effect can be used to make a-axis oriented YBCO films and Josephson junctions or weak links by ion implantation with a narrow bridge mask or focused ion beam technique.

Direct measurement of electron spin-lattice relaxation time of paramagnetic centers and non-linear modulated responses in HTSC

The report consists of two parts. 1. The method is described to measure directly the electron spin-lattice relaxation timeT 1 in high-T c superconductors (HTSC). The technique is based on registering the oscillating longitudinal spin magnetization while saturating the EPR line by an amplitude-modulated microwave field. TheT 1 values of the Cu2+ centers in YBa2Cu3O6+x (x=0.24−0.9) and Y2BaCuO5 materials are measured and found to be about 1.4·10−9 s independently ofx and temperature in the range 77–300 K. Besides much longer relaxation is displayed in degradated materials. The data obtained can be considered as an argument in favor of the “green-phase” origin of the “high-temperature” EPR spectra in the YBaCuO HTSCs. 2. Non-linear responses of the microwave (1010 Hz) and rf (105–107 Hz) absorption in HTSC materials to low-frequency magnetic modulation are studied and found to be quite different in the two frequency ranges. It is shown that at microwaves the effect is caused by non-linear interaction with shielding supercurrents whereas at lower frequencies it is due to jumps of fluxons over potential barriers. The models developed take into account the Josephson weak links and the thermo-assisted flux creep.

Pressure dependence of T c of Y 1−xPr xBa 2Cu 3O 7−δ (0.1≤ x≤0.5, 0≤ δ≤0.5)

The system Y 1− x Pr x Ba 2Cu 3O 7−δ has been investigated for the pressure dependence of the superconducting critical temperature dT c/dP for a range of Pr concentrations x with varying oxygen deficiency δ for each Pr doping level. The variation of both parameters indicates a dependence of the T c suppression by Pr under pressure on the mobile hole concentration in the CuO 2 planes. Furthermore, effects of Josephson weak links in the samples are observed for higher levels of Pr doping and oxygen deficiency.

YBCO thick film fr-SQUID for detection of weak magnetic field signals

RF-SQUIDs based on naturally present grain boundary Josephson weaklinks were fabricated using highly c-axis oriented YBa 2Cu 3O 7−x thick film containing large size grains. Flux noise density of the SQUID was 5×10 -4 Φ o /√Hz at 77K in the white noise region. The SQUID was used to detect weak magnetic signals of low frequency.

DOUBLE STEP BEHAVIOR OF THE CRITICAL CURRENT VERSUS MAGNETIC FIELD IN A BULK JOSEPHSON MEDIUM

This paper discusses the physics of the influence of a strong magnetic field H on the high T c metal ceramic critical current J c . The problem of boundary conditions for a surface current value in hard superconductors is analyzed. It is shown that if Abrikosov vortices in granules are rigidly pinned, the J c vs H dependence has a plateau region of relatively constant critical current. The hysteresis of ceramic critical current is calculated on the basis of the Josephson weak link model.

Current-assisted and thermoassisted non-linearity in radio-frequency absorption in high-Tc materials

A model describing the modulated microwave ( omega /2 pi approximately 1010 Hz) and moderate frequency (104-107 Hz) absorption in high-Tc superconductors is suggested. At microwaves the response of the absorption to the low-frequency modulation is attributed to the interaction of the microwave field with the flux lines oscillating near the pinning centres. The non-linearity is caused by the fluxon displacement due to the shielding supercurrent. It is shown that the experimental data on low-field modulated microwave absorption are consistent with the pinning potential U(u) varies as -(1+cos(au)) similar to that for Josephson weak links. At moderate frequencies the non-linear response is explained by the thermoassisted fluxon jumps over the potential barriers at omega tau <<1, where tau is the characteristic time of the jump.

Large-amplitude Shapiro steps and self-field effects in high-T c Josephson weak links

We demonstrate contiguous Shapiro steps of orders 0 and 1 having amplitudes of 1 mA in a YBa2Cu3O7−δ step-edge junction operated at 38 K. A wide-junction model that includes self-field effects explains why the observed step amplitudes are smaller than expected from the resistively shunted point-junction model. In spite of their reduced amplitudes, the observed steps are suitable for use in a proposed rapidly programmable Josephson voltage standard.

Influence of grain boundary weak links on the nonequilibrium response of YBaCuO thin films to short laser pulses

The transient voltage response in both epitaxial and granular YBaCuO thin films to 80 ps pulses of YAG∶Nd laser radiation of wavelength 0.63 and 1.54 μm was studied. In the normal and resistive states both types of films demonstrate two components: a nonequilibrium picosecond component and a bolometric nanosecond one. The normalized amplitudes are almost the same for all films. In the superconducting state we observed a kinetic inductive response and two-component shape after integration. The normalized amplitude of the response in granular films is up to five orders of magnitude larger than in epitaxial films. We interpret the nonequilibrium response in terms of a suppression of the order parameter by the excess of quasiparticles followed by the change of resistance in the normal and resistive states or kinetic inductance in the superconducting state. The sharp rise of inductive response in granular films is explained both by a diminishing of the cross section for current percolation through the disordered network of Josephson weak links and by a decrease of condensate density in neighboring regions.

Excess current in shunted Josephson weak links

In a variety of Josephson weak links the resistively shunted junction model does not properly describe observed excess currents in the voltage current characteristics. A modification of the model is proposed which more adequately describes experimental data.

Enhanced mode detection for microwaves using a high-impedance YBCO-BiO composite superconductor

We have developed a Josephson type microwave detector using a YBCO-BiO composite superconductor which uses the non-linear effect produced by a complex network of Josephson weak links at the grain boundary (enhanced mode detection). This composite superconductor was prepared by doping YBa 2 Cu 3 O x with Bi 2 O 3 , which causes high normal state resistance in the composite and enhances the Josephson effect at the grain boundary. It was found that the measured responsivity ( R MV ) improved by increasing the normal resistance ( R n ) of the detector through enhanced matching of the incident microwaves and the detector. An R MV of 180 V/W and NEP of 1.4×10 −14 W/Hz 1 2 were obtained with a detector having an R n of 13 Ω. These results suggest the feasibility of a practical detector having an R MV of more than 10 4 V/W which operates at 60 K by the enhanced mode detection of a high- T C superconductor.

Symmetry of the superconducting order parameter in a YBa2Cu3O7- delta epitaxial films.

We propose a way to measure the interference associated with two superconductors with a ${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$ symmetry coupled by a Josephson weak link. An experimental realization shows no interference effects in a superconducting ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ film. The results are consistent with s-wave symmetry.

Fabrication and measurement of high T c weak links

We have fabricated Y 1Ba 2Cu 3O 7−δ (YBCO) superconducting weak links by depositing a perpendicular stripe of YBCO film on top of a 0.25μm–1.5μm wide line of normal metal (e.g. Ag) previously deposited onto a SrTiO 3 substrate. The c-axis orientation of the YBCO film is interrupted over the metal line, creating a disordered grain boundary junction. Measurements of the resistive transition, critical current and current-voltage (IV) characteristics of these structures show definite weak link behavior: the critical current is significantly reduced from that of samples with no metal line present and the IV curves show qualitative agreement with the Ambegaokar-Halperin (AH) model of Josephson weak links with thermal noise.

Low temperature optical response of a single grain boundary in superconducting YBa2Cu3O7−δ thin film

We have investigated the optical response from a single grain boundary in a thin film (∼300 nm) of superconducting YBa2Cu3O7−δ fabricated on bicrystals SrTiO3 substrate. We have studied the temperature, bias current, and magnetic field dependence of the optical response in the temperature range of 15 K≤T≤35 K. We find that the optical response from the single grain boundary in this temperature range can be satisfactorily explained in terms of the optical response of a Josephson weak link. The optical response of the grain boundary is qualitatively similar to that of granular films seen in the earlier studies, indicating that the latter may be dominated by the optical response of grain-boundary weak links.

Effect of noise and capacitance on the dynamical characteristics of high-T c Josephson junctions

The dynamical characteristics of Josephson junctions at finite temperatures are simulated in the resistively shunted junction model including a capacitance and a current source producing white noise. It is shown how the height and the shape of the Shapiro steps as a function of the microwave amplitude are affected by noise as well as by the capacitance. The results of the simulations are found to be in good agreement with recently published experimental data on different types of Josephson weak links fabricated from high-Tc materials.

Surface resistance of grain-aligned YBa2Cu3Ox bulk: Evidence for two kinds of weak link.

We have measured the magnetic-field and temperature dependence of the surface resistance ${\mathit{R}}_{\mathit{s}}$ of grain-aligned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$. Experiments were conducted on ab-plane- and ac-plane-oriented samples, with the dc magnetic field parallel and perpendicular to the sample surface. We explain the nonlinear behavior of ${\mathit{R}}_{\mathit{s}}$ at low field using a model with an array of Josephson weak links and the linear dependence on magnetic field at high field using an oscillatory-vortex-motion-loss model. The model allows a quantitative fit to the experimental data using the lower critical field of the grain, the entry field ${\mathit{H}}_{\mathit{c}1\mathit{J}}$ of weak links, the junction decoupling parameter ${\mathit{H}}_{\mathit{d}}$, and the vortex viscosity as fitting parameters. Two different sets of parameters ${\mathit{H}}_{\mathit{d}}$ and ${\mathit{H}}_{\mathit{c}1\mathit{J}}$ were used to fit the data, suggesting that in grain-aligned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ bulk material at least two kinds of weak link are responsible for low-field losses. An estimation of the Josephson-junction area was obtained from field-modulated microwave absorption. In addition, thermal-excitation and radiation-activation mechanisms for rf losses are compared.

The AC Josephson effect and submillimeter wave mixing with a weak-link array of grain boundaries formed in YBCO film

The authors present a model for a series-parallel array of small metallic (nontunneling) Josephson junctions. The transmission and configuration of pair electrons between small superconducting regions are considered assuming the presence of a strong interaction between the electrostatic energy (2e)/sup 2//2C and the Josephson effect in a network of metallic weak links. The model predicts that the same behavior as that of a single junction is obtained with a superconducting weak-link device comprising multiple small superconducting regions joined by conductive bridges. In an experiment in the 100- and 320-GHz frequency ranges, the constriction of a YBCO film formed with a network of grain-boundary Josephson weak links showed that the AC Josephson effect is the same as for a single Josephson element. It is confirmed that the constriction works as a harmonic mixer at 318 GHz and that a network of high-T/sub c/ superconducting weak links can be used for a practical device operating at submillimeter-wave frequencies.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

YBa2Cu3O7−δ-based, edge-geometry SNS Josephson junctions with low-resistivity PrBa2Cu3O7−δ barriers

Abstract We investigated the properties of all-high- T c edge-geometry Josephson weak links using superconducting YBa 2 Cu 3 O 7−δ (YBCO) electrodes and PrBa 2 Cu 3 O 7−δ (PBCO) normal-metal layers. The fabrication of the weak links involved first ion-milling the base YBCO film using a MgO mask to form the edge and then depositing PBCO and YBCO layers in situ by pulsed-laser deposition. The optimum PBCO films had resistivities of 2 to 4 mΩ cm at room temperature and ≈30 mΩ cm at 4.2 K. Current-versus-voltage ( I–V ) characteristics were qualitatively consistent with the resistively-shunted-junction (RSJ) model. Strong Shapiro steps were present in the I–V characteristics of junctions under microwave irradiation and the critical current of the devices modulated with a Fraunhofer-like dependence as a function of magnetic field. The device parameters exhibited good scaling with area and PBCO thickness. The critical-current density varied exponentially with PBCO thickness, yielding a normal-metal coherence length of 9 nm for the PBCO.

Preparation of superconducting thin films of Bi-Sr-Ca-Cu-O by electron-beam evaporation and a study of RF-SQUID behaviour at liquid nitrogen temperatures

Thin films of Bi-Sr-Ca-Cu-O (2212) have been prepared on a single-crystal MgO (100) substrate by electron-beam evaporation of a doubly calcined powder of Bi-Sr-Ca-Cu-O (2212). After annealing in air at 872 degrees C for 5 min, highly-oriented films with their c axes perpendicular to the substrate are obtained. The films show a sharp superconducting transition with zero resistance at 85 K. The average grain size in these films has been increased to about 50 mu m by applying two-step heat treatment. RF-SQUIDS are fabricated on these films and the SQUID behaviour due to grain-boundary Josephson weak links is observed up to 82.6 K. The spectral density of flux noise of the RF-SQUID is approximately=4*10-4 Phi O Hz-1/2 at 10 Hz and 77 K.

Non-linear response of YBa 2Cu 3O 7−y thin film microstrip resonator at microwave frequencies

Power dependence of microwave surface resistance in c-axis oriented YBa 2 Cu 3 O 7− y thin film microstrip resonators has been studied. Asymmetry in the resonance curves has been observed at microwave power levels above -5 dBm. The observed non-linear response of YBa 2 Cu 3 O 7− y thin film resonators has been discussed on the basis of Josephson's weak links due to the grain boundaries in HTSC materials. The magnitude of critical Josephson's field, H cj , is calculated and its variation with temperature is obtained.